There are many types of evidence supporting evolution, including studies in the molecular biology field, such as DNA, and in the developmental biology field. However, the most commonly used types of evidence for evolution are anatomical comparisons between species. While homologous structures show how similar species have changed from their ancient ancestors, analogous structures show how different species have evolved to become more similar.
Speciation is the change over time of one species into a new species. Why would different species become more similar? Usually, the cause of convergent evolution is similar selection pressures in the environment. In other words, the environments in which the two different species live are similar and those species need to fill the same niche in different areas around the world.
Since natural selection works the same way in these environments, the same types of adaptations are favorable, and individuals with favorable adaptations survive long enough to pass down their genes to their offspring. This continues until only individuals with favorable adaptations are left in the population.
Sometimes, these types of adaptations can change the structure of the individual. Body parts can be gained, lost, or rearranged depending on whether their function is the same as the original function of that part. This can lead to analogous structures in different species that occupy the same type of niche and environment in different locations.
When Carolus Linnaeus first began classifying and naming species with taxonomy, the science of classification, he often grouped similar-looking species into similar groups. This led to incorrect groupings compared to evolutionary origins of the species. Just because species look or behave the same doesn't mean they are closely related.
Analogous structures don't have to share the same evolutionary path. One analogous structure might have come into existence long ago, while the analogous match on another species may be relatively new. They may go through different developmental and functional stages before they are fully alike.
Analogous structures are not necessarily evidence that two species came from a common ancestor. It is more likely they came from two separate branches of the phylogenetic tree and may not be closely related at all.
The human eye is very similar in structure to the eye of the octopus. In fact, the octopus eye is superior to the human's in that it doesn't have a "blind spot." Structurally, that is the only difference between the eyes. However, the octopus and the human are not closely related and reside far from each other on the phylogenetic tree of life.
Wings are a popular adaptation for many animals. Bats, birds, insects, and pterosaurs all had wings. But a bat is more closely related to a human than to a bird or an insect based on homologous structures. Even though all these species have wings and can fly, they are very different in other ways. They just happen to fill the flying niche in their locations.
Sharks and dolphins look very similar due to color, placement of their fins, and overall body shape. However, sharks are fish and dolphins are mammals. This means that dolphins are more closely related to rats than they are sharks on the evolutionary scale. Other types of evolutionary evidence, such as DNA similarities, have proved this.
It takes more than appearance to determine which species are closely related and which have evolved from different ancestors to become more similar through their analogous structures. However, analogous structures themselves are evidence for the theory of natural selection and the accumulation of adaptations over time.